1. Field of the Invention
The present invention relates to a bridge type phase detection device and more particularly, to a bridge type phase detection device which can accurately detect an anomaly of a LCD backlight tube.
2. Description of the Prior Art
FIG. 1A illustrate a prior art tube anomaly detection circuit for LCD backlight, the tube anomaly detection circuit comprising an electronic switch 91, a first and second transformer 921, 922, a first tube 931 and a second tube 932, a first and second capacitor 941, 942, a voltage-detecting protection circuit 95, a current-detecting protection circuit 96, a pulse width modulation (PWM) IC 97. PWM IC 97 and a primary side of first transformer 921 and second transformer 922 couple with the electronic switch 91, and a first end of a secondary side of first and second transformer 921, 922 couple with a first end of first and second tube 931, 932, while a second end of the secondary side of first and second transformer 921, 922 couple with current-detecting protection circuit 96; a second end of first and second tube 941, 942 are grounded, respectively, and a first end of first capacitor 941 couples with the first end of the secondary side of first and second transformer 921, 922 and the first end of first and second tube 931, 932; the second end of first capacitor 941 couples with a first end of second capacitor 942, and a second end of second capacitor 942 is grounded, the place where first capacitor 941 couples with second capacitor 942 is called a detection point 98. Detection point 98 couples with voltage-detecting protection circuit 95, and then couples with current-detecting protection circuit 96 and PWM IC 97 through voltage-detecting protection circuit 95.
In the above mentioned circuitry, voltage-detecting protection circuit 95 detects a voltage value of detection point 98 to find out if first and second tube 931, 932 operate normally; however, the voltage of detection point 98 is derived from dividing an output voltage based on the impedance of first capacitor 941 and that of second capacitor 942; therefore, the voltage is around 1000Vrms during normal operation and rises to 1200V˜1300Vrms when an anomaly occurs, the difference between them is only 20%, which is not easy for voltage-detecting protection circuit 95 to detect any anomaly associated with first and second tube 931, 932.
FIGS. 1B–D illustrate a few prior art tube anomaly detection circuits for LCD backlight. In FIG. 1B, two transformers couple with U-shaped tube 933 to provide power and to detect if U-shaped tube 933 works properly; in FIG. 1C, a single transformer 921 couples with tube 931, 932 to detect if tube 931, 932 work properly; in FIG. 1D, a single transformer 921 couples with U-shaped tube 933 to detect if U-shaped tube 933 works properly; however, all of the tube anomaly detection circuits detect the divided voltage derived from first capacitor 941 and second capacitor 942, it is likely that voltage-detecting protection circuit 95 has difficulties in detecting any anomaly associated with the tubes.
Therefore, the prior art tube current control device mentioned above presents several shortcomings to be overcome.
In view of the above-described deficiency of prior-art tube anomaly detection circuit for LCD backlight, after years of constant effort in research, the inventor of this invention has consequently developed and proposed a bridge type phase detection device in the present invention.